A well known waterproof breathable textile laminate of commerce, sold under the brandname, GORE-TEX, is technically based upon the use of a membrane of hydrophobic, microporous, expanded polytetrafluoroethylene (hereinafter "PTFE") as an essential functional component thereof. For most purposes, the microporous PTFE membrane of the laminate is sandwiched between inner and outer fabric layers, the membrane generally, although not necessarily, being continuously bonded and/or adhered to one or both fabric layers. Such hydrophobic microporous PTFE membranes and the preparation thereof are described in U.S. Pat. No. 3,953,566, Robert W. Gore, issued Apr. 27, 1976 and U.S. Pat. No. 4,187,390, Robert W. Gore, issued Feb. 8, 1980. The preparative method broadly comprises the uniaxial or biaxial stretching of an unsintered highly crystalline PTFE sheet, prepared by paste extrusion, at a rate exceeding 10% per second while maintaining the temperature thereof at between about 35.degree. C. and the crystalline melt point of the polymer. The resulting membranous PTFE product of this process has a microstructure characterized by nodes interconnected by fibrils and is possessed of an interesting and useful combination of properties which befits it for use in the preparation of waterproof breathable textile systems. More specifically, the hydrophobic, microporous, expanded PTFE membranes of the above Gore patents are sufficiently hydrophobic and the micropores small enough such that the membrane can function as a barrier to the passage of bulk water therethrough at significant hydrostatic pressures. On the other hand, due to the microporous character thereof, said membranes also possess the capacity to allow diffusion of water vapor therethrough. Thus, these desirable properties of waterproofness and water vapor transmissibility or "breathability" are imparted to textile laminates in which such a membrane comprises a component layer.
Despite its relative success in the marketplace, the waterproof breathable textile laminate systems based on the hydrophobic microporous expanded PTFE membrane of the above-identified Gore patents are, nevertheless, possessed of certain deficiencies. Firstly, such expanded PTFE membranes are relatively expensive. Secondly, because waterproof breathable textile laminates utilizing such microporous expanded PTFE membranes are necessarily constructed by some form of physical lamination of the previously prepared PTFE membrane to at least one previously prepared fabric layer, equipment and operative techniques must be provided to handle the membrane and the fabric, to properly index them and to secure the PTFE membrane layer to the fabric layer. Thus, the techniques used to prepare such laminates are generally substantially more complex, arduous and expensive as compared to conventional liquid coating and curing techniques known in the fabric coating art. Thirdly, PTFE materials are generally known to be of adhesion resistant character. Therefore, where it is desired to prepare a waterproof breathable textile laminate by means of a continuous bonding of the PTFE membrane layer to a fabric layer, such as by interposition of a continuous adhesive layer between the microporous expanded PTFE membrane layer and the fabric layer, the resulting bond strength, albeit usually adequate, is generally relatively low and the selection of a suitable adhesive which can accomplish the necessary bonding without substantial adverse affect upon the breathability of the system can be a problem. Another problem associated with the use of the microporous expanded PTFE membranes in textile laminate systems resides in the finding that such membranes, over a period of use, can crack sufficiently as to provide sites for bulk water leakage or seepage therethrough. Apparently, the many micropores necessarily embodied in the polymer matrix can act as crack propogation loci and this, coupled with the crystalline nature of the polymer, result in a membrane whose flexure life is somewhat limited. Finally, the expanded microporous membranes of the above-identified Gore patent are apparently susceptible to significant loss of their waterproof characteristics when contacted with such surface active agents as are inherently contained in human perspiration. Obviously, this can be a serious detriment where textile laminates employing these membranes are employed as garment materials. This problem, as well as a solution therefor, is disclosed in U.S. Pat. No. 4,194,041, Gore et al., issued Mar. 18, 1980. The solution therein disclosed resides in the interposition of a continuous, hydrophilic, water vapor transmissible layer between the microporous hydrophobic membrane surface and the surfactant source. Thus, with respect to garments fabricated with a waterproof breathable textile laminate prepared in accordance with the aforementioned Gore et al. patent, the microporous hydrophobic membrane layer faces the exterior of the garment while the continuous hydrophilic layer faces the interior of the garment. In this role, the continuous hydrophilic layer functions as a barrier to the surfactant contaminants in human perspiration and prevents contact thereof with the hydrophobic microporous PTFE membrane, thereby to preserve the waterproof character of the membrane. As disclosed, the continuous hydrophilic layer of the construction can be in the nature of a hydrophilic polyurethane based on a reactive crosslinkable prepolymer having an isocyanate terminated branched polyoxyethylene backbone. Attachment of the continuous hydrophilic layer to the microporous hydrophobic membrane can be discontinuous, such as in the nature of sewing or adhering together of the edges of the respective layers by thread or adhesive. The use of a continuous adhesive bond between the hydrophobic and hydrophilic layers is apparently believed by the patentees to be potentially detrimental to the water vapor transmission properties of the laminate and so is not discussed or disclosed. Where continuous bonding of the respective layers of the laminate is desired, patentees disclose a technique whereby direct bonding of the one layer to the other is achieved. Said technique involves the casting of the hydrophilic layer directly onto the surface of the microporous hydrophobic membrane layer with application of sufficient hydraulic pressure as to force the hydrophilic layer into the surface voids of the hydrophobic layer. Thus, while the Gore et al. patent may provide a second generation solution for the surfactant contamination problem disclosed to exist with respect to garment applications involving the first generation hydrophobic microporous expanded PTFE membranes disclosed in U.S. Pat. Nos. 3,953,566 and 4,187,390, it is obvious that the solution is achieved at the expense of added complexity and, of course, cost.
In U.S. Pat. No. 4,532,316, Robert L. Henn, issued Jul. 30, 1985, there is generally disclosed a phase separated polyurethane prepolymer having hard and soft segments and elastomers prepared therewith. The prepolymer comprises the product of reaction of (a) a polyol having a number average molecular weight of between 600 and 3500, (b) a polyisocyanate having a functionality of at least 2, and (c) a chain extender having a molecular weight of no greater than 500, these components of the prepolymer being present within a specified range of proportions. It is further disclosed that, where the polyol component employed is poly(oxyethylene) glycol, the prepolymer can be formed into cured films having superior moisture vapor transmission properties and that such prepolymer can thus be formed as a composite with various textiles for use in rain protective garments. Example 10 of the patent discloses the preparation of breathable coated textiles by melt coating of a fabric with poly(oxyethylene) glycol based prepolymers prepared in accordance with the invention followed by moisture curing of the coating. Said breathable coated textiles were found to be durably waterproof under scoring and flexing tests and were stated by patentee as being suitable for use in rain protective wear. In Example 11, a hydrophilic phase separated non-porous film of hydrophilic polyurethane was directly bonded to an expanded PTFE membrane in the manner of the aforementioned U.S. Pat. No. 4,194,041. A poly(oxyethylene) glycol based prepolymer different from those employed in Example 10 was utilized and roll coated onto the PTFE membrane under pressure, followed by ambient moisture curing of the coating. The resulting PTFE/polyurethane composite was discontinuously adhered to a textile fabric to form a breathable textile laminate, the PTFE layer being sandwiched between the fabric and polyurethane layers. The patentee discloses this textile construction as being suitable for the fabrication of excellent rain protective wear therefrom. From the disclosure it is suggested that Henn regards the attainment of acceptable moisture vapor transmissions rate for clothing purposes as being absolutely dependent upon the use of a poly(oxyethylene) glycol based prepolymer and, to a lesser extent, upon the extent of phase separation attained between the hard and soft segments of the cured polymer.
In accordance with the present invention there is provided a waterproof breathable polyurethane membrane whose waterproofness and breathability properties are not dependent upon the presence of microporosity and in which at least several of the problems related to the known microporous membranes of the prior art have been solved or substantially ameliorated. In another aspect of the invention, the polyurethane membranes hereof are disposed over porous substrates, particularly as coatings applied to fabrics, thereby to confer waterproofness and breathability to such substrates.